An MRI (magnetic resonance imaging) apparatus is a medical diagnostic imaging apparatus that produces magnetic resonance in a nuclear spin within an arbitrary cross section passing transversely across a test subject, and obtains a tomographic image in the cross section based on the magnetic resonance signals being generated. When a test subject placed in a static magnetic field is irradiated with an RF magnetic field from a high frequency coil (RF coil), together with application of a gradient magnetic field, a nuclear spin within the test subject, a nuclear spin of hydrogen atom, for instance, is excited. When the nuclear spin being thus excited restores to an equilibrium state, a magnetic field of circularly polarized wave occurs as a magnetic resonance signal. The RF coil detects this signal, and the signal is subjected to a signal processing so as to create an image representing a distribution of atomic nuclei of hydrogen within a living body.
There are some types for this RF coil, including a transmission coil exclusively used for irradiation of the RF magnetic field, a reception coil exclusively used for receiving the magnetic resonance signal, and a transceive coil used for both transmission and reception. Various kinds of coils are developed for each of those types, in order to efficiently obtain an image of high quality. For example, when the nuclear spin within the test subject is excited, it is necessary to have a homogeneous distribution of the RF magnetic field. It is desirable that a degree of the homogeneity is within 70% with respect to a maximum value of the distribution of the RF magnetic field in the imaging region. This is because if high inhomogeneity exists in the distribution of the RF magnetic field, a difference may occur in the excited state of the nuclear spins, depending on portions in the test subject, and unevenness of contrast and/or an artifact may be generated in the image being obtained. As the RF coil having this kind of homogeneous distribution of irradiation strength, there are known cylindrical RF coils such as a birdcage coil (e.g., see Patent Document 1 and Non Patent Document 1), and TEM coil (e.g., see Non Patent Document 2). The birdcage coil is made up of two cylindrical ring conductors, plural linear rung conductors, and plural capacitors. Here, the ends of the rung conductors are connected to the ring conductors, and the capacitors are placed on the ring conductors or on the rung conductors.
Furthermore, it is necessary to enhance the irradiation efficiency. As a method for enhancing the irradiation efficiency, there is a method of quadrature phase detection (QD: Quadrature Detection) (see Patent Document 2 and Non Patent Document 3, for instance). The QD method uses two RF coils for irradiation of high frequency magnetic fields being orthogonal to each other, and irradiation of the RF magnetic fields is performed in such a manner that a phase difference in time phase is 90 degrees between the respective irradiation of the RF magnetic fields from the RF coils. The QD method allows the circularly polarized wave field for exciting the nuclear spin of the hydrogen atom to be irradiated with a high degree of efficiency, and therefore, the irradiation strength can be enhanced theoretically by √2, compared to the case of irradiation by one RF coil. If it is converted into irradiation power, only a half of the power is required, and therefore the irradiation efficiency is doubled. When the birdcage coil or the TEM coil is employed, two feeding ports used for the irradiation are arranged at the positions orthogonal to each other, thereby enabling irradiation of the RF magnetic fields according to the QD method, just by one coil.
Since the trunk of the test subject has a shape similar to an elliptic cylindrical shape, an elliptic cylindrical shape rather than a cylindrical shape enables enhancement of a ratio which represents occupancy of the test subject within the coil (filling factor), thereby improving the irradiation efficiency of the RF coil. Accordingly, an elliptical birdcage coil having an elliptic cylindrical shape is developed (see Patent Document 3, Non Patent Document 4, and Non Patent Document 5, for instance).